Tritium release from FLiBe is a significant safety issue in Molten Salt Reactors (MSRs) including both Fluoride Salt-Cooled High-Temperature Reactors (FHRs) and dissolved fuel MSRs, and fusion reactors. Tritium is formed in FLiBe through neutron interactions with both lithium and beryllium. The tritium generally either exists in the salt as tritium fluoride (TF), a dissolved ion (T+), or as dissolved tritium gas (HT or T2). Shifting the redox potential of the fluoride salt to a more reducing condition shifts the chemical equilibrium away from tritium-fluoride. Metallic beryllium contact has been shown to effectively reduce TF to T+ in FLiBe. Excess beryllium in the salt will keep the FLiBe TF concentration below 20 ppt. Tritium gas has a very low solubility in FLiBe. The equilibrium partial pressure of tritium gas over FLiBe with 1 ppm T2 is 105 Pa.
The tritium will transport along with the salt. The generated tritium can be trapped by the carbonaceous materials in the primary loop, escape through the primary coolant surface into the cover gas, permeate through the reactor vessel or piping, or permeate through the heat exchanger tubing. The large surface area and thin tubing walls combined with the turbulent mixing within the heat exchanger makes tritium escape through the heat exchanger tubes a significant tritium escape mechanism. Tritium has been calculated and experimentally demonstrated at the Molten Salt Reactor Experiment (MSRE) at the Oak Ridge National Laboratory to significantly transfer from FLiBe under turbulent flow through heat exchanger tubes.
The calculated tritium production rate at the MSRE was 54 Ci/day, and the observed disposition of tritium, not including retention in the off-gas system, amounted to 80% of this production rate: 48% discharging from fuel off-gas system, 2% discharging from coolant off-gas system, 7% discharging in coolant radiator air, 9% appearing in cell atmosphere, and 14% going into the core graphite. Most of the remainder was probably held up in oil residues in the fuel off-gas systems. Further information and attribution can be found in the references listed at the end of the specification.
Tritium can be a hazardous radioactive contaminant under the above described and other conditions, but if it can be sequestered, tritium would be a valuable commodity, being useful for various applications, particularly as the parent isotope for 3He for which there is currently a global shortage. There has been heretofore a need for an effective and practical mechanism to strip tritium from FLiBe that is used in nuclear power plants.